CIDR Notation Reference Table — Complete IPv4 Subnet Prefix Guide

This CIDR reference table lists all IPv4 subnet prefixes from /0 to /32 with their corresponding subnet masks, total host counts, usable host counts and typical use cases. Bookmark this page for instant reference during network design, firewall configuration, cloud VPC setup, or CCNA/Network+ exam revision.

What is CIDR Notation?

CIDR (Classless Inter-Domain Routing) is the modern standard for representing IP addresses and their associated subnet masks. The number after the slash (e.g. /24) indicates how many bits of the 32-bit IP address are used for the network portion. The remaining bits are available for host addresses. CIDR replaced the older Class A/B/C system in 1993, allowing far more flexible and efficient IP address allocation.

Key CIDR Prefixes Every Network Engineer Should Know

• /8 — Class A — 16,777,216 hosts — large ISP or enterprise allocations
• /16 — Class B — 65,536 hosts — large corporate networks
• /24 — Class C — 254 usable hosts — standard office LAN segment
• /25 — 126 usable hosts — splitting a /24 into two equal halves
• /26 — 62 usable hosts — department-level segments
• /28 — 14 usable hosts — small server segments
• /30 — 2 usable hosts — point-to-point WAN links
• /31 — 2 addresses, no broadcast — RFC 3021 point-to-point links
• /32 — Single host — loopback addresses and static host routes

CIDR in Cloud Networking (AWS, Azure, GCP)

Cloud providers use CIDR notation extensively for VPC (Virtual Private Cloud) design. AWS recommends using /16 for your VPC (65,536 addresses) and /24 for individual subnets. Always plan your CIDR blocks carefully — once a VPC is created, its CIDR range cannot be changed without recreation. Leave space in your address plan for future subnet additions.